A light-emitting diode is a semiconductor diode that emits light when electrically biased in the forward direction of a p-n junction. A typical vertical LED structure may include an n-electrode, a substrate, an n-type layer, a multiple or single quantum well region, a p-type layer, and a p-electrode.
A challenge for LED device design is contacting the emission face of the LED. Metal structures typically used as electrodes may obstruct the transmission of light emitted from the active light-emitting region of the LED. Further, the low conductivity of p-GaN and its alloys may necessitate lateral current spreading schemes. In general, the greater the current that is flowing through the LED, the brighter the light emitted. However, current spreading across the active light-emitting region must be carefully controlled. Too much current or highly localized current may cause damage to the active light-emitting material. In contrast, not enough current, uniformly reaching the LED material, may lead to inefficiencies in light emission.
Several methods have been implemented to contact the emission face of the LED. Some methods involve the use of a transparent conductive material such as, for example, indium tin oxide. Other methods may use metal contacts of various types including very thin nickel-gold films to contact the LED emitting face.
Turning to prior art FIG. 7, an example of prior art LED 702 uses an indium tin oxide (ITO) coating 712 on top of top contact layer 714. A metal pad 710 is attached on the top face 716 of ITO coating 712 thus forming an electrode to top contact layer 714. A disadvantage of this method is that metal pad 710 blocks a percentage of the emitted light. Further, ITO coating 712 has been known to degrade over time for UV LEDs.
Another example of a prior art LED 704 comprises an ITO coating 722 disposed on top contact layer 724 and an ITO pad 720 affixed to ITO coating 722. This top electrode method allows for better transmission of the light. However, bonding ITO pad 720 to ITO coating 722 may be difficult and may lead to early failure of the device. Further, example prior art LED 704 may have the disadvantages discussed above.
Yet another example of a prior art LED 706 comprises a thin film of Ni/Au 732 disposed on top contact layer 734, with a metal pad 730 affixed to the thin film of Ni/Au 732. The Ni/Au film 732 may block some light transmission, degrading LED efficiency in addition to the obstruction of light by metal pad 730.
What is needed then, is a new structure and method of contacting LEDs that overcomes the above described shortcomings in the prior art.